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Communicating with hypersonic vehicles in flight

Researchers propose a potential new way to maintain communication with re-entering spacecraft and other vehicles by matching resonance of the antenna with that of the surrounding hypersonic sheath.

As a spacecraft enters Earth's atmosphere it becomes encircled by hot ionized air - a so-called plasma sheath - that cuts off radio communications. "When a re-entry vehicle is unable to be connected, the only thing you can do is pray for it," said Xiaotian Gao, a physicist at the Harbin Institute of Technology in China.

Gao and his colleague Binhao Jiang have proposed a new way to maintain communication with spacecraft as they re-enter the atmosphere. The approach might also be applied to other hypersonic vehicles such as the military aircraft of the future and ballistic missiles. They discuss their approach in the Journal of Applied Physics.

Essentially, under certain special conditions, a plasma sheath can actually enhance the radiation from a communication antenna, and Gao and his colleagues believe it would be possible to replicate these special conditions in ordinary hypersonic flight by redesigning the antenna.

The researchers first analysed earlier experiments and found that the special signal enhancement could be explained by a resonance, or matched electromagnetic oscillations, between the plasma sheath and the surrounding air. They propose adding a 'matched layer' to ordinary communication antennas to create the desired resonant conditions during normal hypersonic flight.

The matched layer works because it acts as like a capacitor in the antenna circuit; the plasma sheath, on the other hand, acts like an inductor. Paired, they form a resonant circuit. "Once the resonance is reached, the energy can be exchanged between them steadily and without loss, like real capacitance and inductance do in a circuit," Gao said. "As a result, the electromagnetic radiation can propagate through the matched layer and the plasma sheath as if they do not exist."

For the resonance to work, the thickness of the matched layer and the plasma sheath must be smaller than the wavelength of the electromagnetic waves used to communicate, so the approach would be ineffective if the antenna frequency were too high.

The properties of the plasma sheath can change during flight, but Gao and his colleagues believe their matched layer can adjust for these changes if it is made from a material whose electromagnetic properties can be tuned with an electrical signal.

"We don't need to know exactly the properties of the plasma layer, but we need to know the ranges for these properties. The matched layer will be adjusted by an automatic control system, so we only need to know the ranges to make sure this whole system can work appropriately," Gao adds.